In electrophotography an image comprising an electrostatic field pattern, usually of non-uniform strength (also referred to as an electrostatic latent image), is formed on an insulative surface of an electrophotographic element comprising at least a photoconductive layer and an electrically conductive substrate. The electrostatic latent image is usually formed by imagewise radiation-induced dissipation of the strength of portions of an electrostatic field of uniform strength previously formed on the insulative surface. Typically, the electrostatic latent image is then developed into a toner image by contacting the latent image with an electrographic developer. If desired, the latent image can be transferred to another surface before development.
In latent image formation the imagewise radiation-induced dissipation of the initially uniform electrostatic field is brought about by the creation of electron/hole pairs, which are generated by a material, often referred to as a photoconductive or charge-generation material, in the electrophotographic element in response to exposure to imagewise actinic radiation. Depending upon the polarity of the initially uniform electrostatic field and the types of materials included in the electrophotographic element, part of the charge that has been generated, i.e., either the holes or the electrons, migrates toward the charged insulative surface of the element in the exposed areas and thereby causes the imagewise dissipation of the initial field. What remains is a non-uniform field constituting the electrostatic latent image.
Several types of electrophotographic recording elements are known for use in electrophotography. In many conventional elements, the active photoconductive or charge-generation materials are contained in a single layer. This layer is coated on a suitable electrically conductive support or on a non-conductive support that is overcoated with an electrically conductive layer. In addition to single-active-layer electrophotographic recording elements, various multi-active electrophotographic recording elements are known. Such elements are sometimes called multi-layer or multi-active-layer elements because they contain at least two active layers that interact to form an electrostatic latent image.
A class of photoconductive materials useful in the aforementioned single-active-layer and multiactive elements is the class of perylene pigments, particularly perylene-3,4,9,10-tetracarboxylic acid imide derivatives. Representative examples of patents pertaining to such perylene photoconductive pigments include, U.S. Pat. No. 4,578,334, issued Mar. 25, 1986, which describes multi-active electrophotographic recording elements that contain, as photoconductive materials, certain crystalline forms of N,N'-bis(2-phenethyl)perylene-3,4:9,10-bis(dicarboximide) characterized by particular spectral absorption and x-ray diffraction characteristics; U.S. Pat. No. 4,714,666, issued Dec. 22, 1987, which describes single-active-layer electrophotographic elements and multi-active elements containing, as photoconductive materials, asymmetrically substituted perylene-3,4,9,10-tetracarboxylic acid imide derivatives, and U.S. Pat. No. 4,792,508, issued Dec. 20, 1988, which describes multi-active elements that contain as photoconductive materials, mixtures of cis- and trans-naphthimidazole perylenes.
Unfortunately, electrophotographic recording elements of the prior art which contain photoconductive perylene materials have typically suffered from one or more disadvantages that have significantly restricted their use. For example, vacuum sublimation (also known as vacuum deposition) is frequently required to deposit photoconductive perylene pigments in a crystal form suitable for high speed electrophotographic elements. Thus, U.S. Pat. No. 4,578,334 describes a process wherein a perylene pigment is deposited by vacuum sublimation in the form of an amorphous layer and is thereafter converted to photoconductive crystalline form by contacting the layer with an appropriate liquid composition. Vacuum sublimation, however, is a batch process which makes production scale runs quite costly and thin sublimed films are fragile and susceptible to damage until they can be protected by a more durable overcoat.
To avoid the disadvantages inherent in forming photoconductive perylene pigment layers using vacuum sublimation techniques and the fragile nature of such layers; electrophotographic layers have been coated from liquid coating compositions comprising finely-divided photoconductive perylene pigments dispersed in solvent solutions of polymeric binders. See, for example, U.S. Pat. No. 4,714,666. Electrophotographic layers coated from such dispersions are more resistant to abrasion and more durable than the layers formed by vacuum sublimation but, these advantages are obtained at the expense of a considerable loss in electrophotographic speed. U.S. Pat. No. 4,714,666 illustrates this point very well since such loss in speed is evident from a comparison between electrophotographic speeds reported in the working examples for electrophotographic recording elements containing perylene pigments in dispersion coated layers and those elements containing such pigments in vacuum deposited layers.
Also, dispersion coated layers containing photoconductive perylene pigments provided by conventional prior art methods are deficient in several respects, for example, the pigments have a relatively large particle size and are poorly dispersed in the binder and do not form homogeneous layers having the uniform distribution of fine particles that is necessary to achieve optimum electrophotographic speed. In addition, such layers often contain agglomerates of individual pigment particles and such agglomerates detrimentally affect the image quality of copies formed with electrophotographic elements containing the layers.
Conventional prior art procedures normally used for forming dispersion-coated layers typically involve mixing the components of a liquid coating composition, for example, a dispersion of photoconductive perylene pigment in a solvent solution of polymeric binder, in a suitable mixing device such as a ball mill or a paint shaker. As previously indicated, such conventional procedures do not adequately disperse the pigment particles and frequently form the aforementioned particle agglomerates. Moreover, prolonged mixing of the photoconductive perylene pigment in a device such as a ball mill can damage the pigment structurally so that electrophotographic performance is detrimentally affected.
It is an objective of this invention to provide electrophotographic recording elements that comprise photoconductive perylene pigments and have excellent photosensitivity, for example, photodischarge speed and dark decay, but do not require vacuum sublimation techniques to achieve such photosensitivity. It is also an objective of this invention to provide electrophotographic recording elements comprising layers containing photoconductive pigments dispersed in polymeric binders, which layers are highly resistant to abrasion and exhibit good durability.